Hematopoietic stem cells (HSCs) are a rare population of cells that can self-renew and differentiate into all blood cell types. They form the basis of bone marrow transplantation for treatment of leukemia and other cancers. However, the clinical applications of HSCs are severely hampered by our limited understanding of the extracellular and intracellular signals that govern HSC cell fates and by the difficulty in ex vivo expansion of these cells. Recently I identified day 15 mouse fetal liver CD3+Ter119- cells as a novel cell population that supports HSC expansion. From these cells I identified insulin-like growth factor 2 (IGF-2) and Angiopoietin-like protein 2 (Angptl2) as the factors that stimulate the ex vivo expansion of HSCs. In combination with other growth factors, Angptl2 stimulates a greater than 20-fold expansion of HSCs in 10 days of culture. Angptl2 is largely unstudied and its receptor(s) and signal transduction are unknown. I hypothesize that Angptl2 interacts with a currently unknown receptor on the surface of HSCs, and this results in activation of specific intracellular signaling pathway(s) and induction of transcription of targets gene(s) that are required for self-renewal or survival of HSCs. To this end, will test the hypotheses that Angptl2, IGF-2, and other growth factors stimulate HSC expansion by inducing their self-renewal and/or survival, and dissect the specific intracellular signal transduction pathway(s) and transcriptional activation induced by Angptl2, in both cell lines and in HSCs. Moreover, I will use expression cloning to isolate the Angptl2 cell surface receptor(s). This work will provide new insights into a novel link between Angptl2, its membrane receptor(s), and the regulation of HSC expansion. Through this proposed research, the applicant will receive training in advanced molecular biology, stem cell biology, protein biochemistry, and system biology in order to gain the requisite skills and knowledge to become an independent investigator.